The present preferred embodiments relate to methods for the production of solenoidal magnets having supported outer coils, and solenoidal magnets so produced.
The present preferred embodiments particularly relate to such solenoidal magnets for use as a magnetic field generator in a Magnetic Resonance Imaging (MRI) system. In particular, the preferred embodiments relate to such magnets formed of superconductive wire.
In known magnet arrangements, a solenoidal magnet typically comprises end coils of a relatively large number of turns, and hence cross-section and a number of inner coils of smaller number of turns and hence cross-section. Conventionally, an accurately machined former, such as a cylindrical aluminum former, is provided with appropriately shaped recesses into which wire is wound to form the coils. The coils may be impregnated with a thermosetting resin, either by wet-winding, in which a wire is passed through a bath of resin before being wound onto the former, or the coils may be wound dry, with the completed coils and former later being impregnated in a bath of resin.
Alternatively, arrangements of molded coils are known. In these arrangements, wound coils are placed into resin baths, and the finished coil impregnated with resin within the resin bath. The resin is then cured, and a solid coil embedded in resin is produced. These molded coils are then assembled into a magnet, for example by clamping onto a former or other mechanical support structure.
Actively-shielded magnets are also provided with shield coils, which are outer coils of greater diameter than the end coils and the inner coils of the solenoidal magnet. Such shield coils are typically wound into accurately-machined metal journals, and these journals are attached to the former using a number of webs spaced circumferentially around the journals.
These known arrangements suffer from certain drawbacks.
In use, the shield coils are subject to large forces, due to interaction of the coils with the magnetic fields produced. Some of these forces, for example the force known as the body force, act axially. The body force typically urges the shield coils away from a center of the magnet, although the body force may act to urge the shield coils towards the center of the magnet, and towards each other, depending on the design of the magnet. Other forces, for example the so-called hoop stress, act radially, tending to expand the coil to a larger diameter. The reaction force through the webs, required to oppose these axial and radial forces, puts strain onto the former, requiring the former and webs to be large and heavy to resist these forces. These forces may cause the shield coils to move relative to the journal. Such movement may cause localized heating of the shield coils, which in superconducting magnets may lead to a quench.
The forces acting on the shield coils may cause the journal, or its support structure, to flex. Due to such flexure, the force reaction path resisting the forces on the coil then acts essentially at the radially inner edge of the shield coil, and the reaction forces are borne by a limited surface area of the coils. This may cause deformation of the coils themselves, which may also lead to quench in a superconducting coil. Conventionally, large and heavy support structures are provided to resist the forces acting on the outer coils.
An example arrangement for retention of shield coils is described in U.S. Pat. No. 5,237,300, incorporated herein by reference. In that document, the shield coils are said to be mounted within coil support cylinders.
An accurately-machined journal, together with its support structure, as conventionally used, is large, heavy, expensive, and is only available from a limited number of suppliers. Transport costs from the journal factory to the magnet winding facility may be significant. Storage of the large journal may be difficult and costly.